Aquatic macrophytes (Spirogyra porticalis and Nymphaea L.) as substrates for biofuel production: potentials and challenges

Abstract

Green fuel represents one of the most promising means of sustainably replacing liquid fuels, and freshwater macrophytes has become one of the auspicious substrates for biofuel production due to high cellulose and hemicellulose contents. In this study, the use of Spirogyra porticalis and Nymphaea alba Linn for biodiesel, bioethanol and biogas production was investigated with other feedstocks (rice husk and biochar) using dry and wet extraction methods. Spirogyra porticalis and Nymphaea alba Linn were harvested from a freshwater lake in Ndufu Echara in Ikwo Local Government, Ebonyi State, Nigeria and thoroughly washed. Biodiesel, bioethanol and biogas production by an alkali-catalysed transesterification using mechanical and chemical methods, acid and base simultaneous saccharification and fermentation, and anaerobic digestion was carried out, respectively. Results showed that freshwater macrophytes produced more biodiesel compared to rice husk and biochar. S. porticalis and Nymphaea L. yielded higher biodiesel using chemical and mechanical methods, respectively. Quality evaluation of the diesel oil according to ASTM standard revealed that chemical method was the most proficient for biodiesel production because of its proximity to ASTM D6751 standard. For bioethanol, acid hydrolysis (1M HCl) produced more bioethanol than the base (1M NaOH) with Nymphaea L. yielding 60% ethanol. However, the macrophytes did not generate biogas after 30-d of anaerobic digestion compared to other feedstocks. Further analysis revealed sample acidification and inhibitions on the anaerobes responsible for anaerobic digestion (p<0.05). This suggests that acidification and/or toxicity may be a major factor associated with anaerobic digestion and biogas production from aquatic macrophytes. Thus, optimization and/or combined pre-treatment methods (biological, mechanical, enzymatic) are recommended. This suggests that Spirogyra porticalis and Nymphaea alba Linn could be major substrates for biofuel production, and tropical freshwater ecosystems can be good for their large-scale cultivation